A horizontal bias in human visual processing of orientation and its correspondence to the structural components of natural scenes.

Many encoding mechanisms and processing strategies in the visual system appear to have evolved to better process the prevalent content in the visual world. Here we examine the relationship between the prevalence of natural scene content at different orientations and visual ability for detecting oriented natural scene content. Whereas testing with isolated gratings shows best performance at horizontal and vertical (the oblique effect), we report that when tested with natural scene content, performance is best at obliques and worst at horizontal (the horizontal effect). The present analysis of typical natural scenes shows that the prevalence of natural scene content matches the inverse of this horizontal effect pattern with most scene content at horizontal, next most at vertical, and least at obliques. We suggest that encoding of orientation may have evolved to accommodate the anisotropy in natural scene content by perceptually discounting the most prevalent oriented content in a scene, thereby increasing the relative salience of objects and other content in a scene when viewed against a typical natural background.